Structure of Oceanic Core Complexes: Constraints from Seafloor Gravity Measurements Made at the Atlantis Massif Scott L

GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 8, 1446, doi:10.1029/2003GL017126, 2003 Structure of oceanic core complexes: Constraints from seafloor gravity measurements made at the Atlantis Massif Scott L. Nooner, Glenn S. Sasagawa, Donna K. Blackman, and Mark A. Zumberge Scripps Institution of Oceanography, La Jolla, California, USA Received 3 February 2003; revised 25 March 2003; accepted 28 March 2003; published 30 April 2003. [1] Using the DSV Alvin, the relative seafloor gravimeter are rafted to the surface. Therefore, fault rocks and ultramafic ROVDOG was deployed at 18 sites on the Atlantis Massif rocks that have been sampled on or near the Atlantis Massif (located at the ridge-transform intersection of the Mid- and other hypothesized core complexes [Dick et al., 1991; Atlantic Ridge and the Atlantis Transform Fault near 30°N, Cannat et al., 1995; Karson, 1999; MacLeod et al., 2002; 42°W). These data along with previously collected shipboard Tucholke and Lin, 1994; Cann et al., 1997; Tucholke et al., gravity and bathymetry provide constraints on the density 2001; Blackman et al., 2001] are congruent with this idea. structure of this oceanic core complex. A series of quasi 3-D [3] Seismic evidence of a detachment fault on the African forward models suggests that symmetric east and west- plate capable of exhuming lower crustal or upper mantle dipping density interfaces bound the core of the massif with material has been shown by Ranero and Reston [1999]. dip angles of 16°–24° in the east and 16°–28° in the west, Uplift of high-density rock in this way might explain the creating a wedge with a density of 3150–3250 kg/m3. The dip presence of the observed gravity high on the Atlantis angle in the east is steeper than that of the surface slope, Massif. However, Campbell and John [1996] gave evidence suggesting that the detachment fault surface does not coincide for emplacement of a synextensional dike-like pluton with the density boundary. The resulting low-density layer is beneath a detachment fault within the Colorado extensional interpreted as a zone of serpentinization. INDEX TERMS: corridor to explain the gravity high observed there. We use 3010 Marine Geology and Geophysics: Gravity; 3094 Marine new seafloor gravity data in combination with existing sea Geology and Geophysics: Instruments and techniques; 1219 surface gravity data to constrain a suite of 3-D forward Geodesy and Gravity: Local gravity anomalies and crustal models in order to place bounds on the geometry and value structure; 8150 Tectonophysics: Plate boundary—general (3040); of the anomalous density in the exposed oceanic core 9325 Information Related to Geographic Region: Atlantic Ocean. complex at Atlantis Massif. Citation: Nooner, S. L., G. S. Sasagawa, D. K. Blackman, and M. A. Zumberge, Structure of oceanic core complexes: Constraints 1.2. Seafloor and Sea Surface Gravity Acquisition from seafloor gravity measurements made at the Atlantis Massif, [4] We collected the seafloor gravity data with our sea- Geophys. Res. Lett., 30(8), 1446, doi:10.1029/2003GL017126, floor gravimeter, ROVDOG (Remotely Operated Vehicle 2003. deployable Deep Ocean Gravimeter), during the Nov.– Dec., 2000 cruise of the R/V Atlantis as one component of MARVEL 2000. The ROVDOG was carried outside DSV 1. Introduction Alvin and placed on the sea floor while an operator inside 1.1. Background the Alvin controlled the instrument and observed the data [2] This study was focused on Atlantis Massif, which is collection in real time. After an observation time of 10–20 located at the eastern inside corner of the intersection minutes the instrument was retrieved and transported to the between the Atlantis Transform Fault and the Mid-Atlantic next site. A seafloor survey taken with a single meter at an Ridge (MAR) at 30°N, 42°W. Spreading parallel corruga- average depth of 300 m has demonstrated a precision of 28 tions on the domal surface (Figure 1) are reminiscent of mGal or better. Further technical details may be found in surface morphology at some continental core complexes in Sasagawa et al. [2003]. the Basin and Range and have been interpreted as character- [5] The locations of the gravity dive sites (shown in istics of a detachment fault surface [Davis and Lister, 1988; Figure 1) form an approximately spreading-parallel line. Cann et al., 1997]. Similar features have been seen at two During each of the dives, the Alvin gathered rock samples fossil massifs along the Atlantis Fracture Zone [Blackman et and stopped for gravity measurements. On average, the 18 al., 1998; Cann et al., 1997], at other places along the MAR gravity sites were spaced 557 meters apart during each of [e.g., Cannat et al., 1995; MacLeod et al., 2002; Tucholke et the four dives1. The sea surface gravity data that we used as al., 2001] and elsewhere [e.g., Ranero and Reston, 1999]. part of the modeling are described in Blackman et al. The current accepted models are that these topographic highs [1998]. The uncertainty in these data are 1.8 mGal as are oceanic core complexes that form when the extension of indicated by the standard deviation in track crossing misfits. the crust is taken up by faulting along a detachment fault The location of the line is shown in Figure 1. rather than by plate accretion [Dick et al., 1991; Mutter and Karson, 1992; Tucholke and Lin, 1994]. As the crust extends via this low angle fault, lower crustal and upper mantle rocks 2. Data Reduction for Seafloor Gravity [6] We corrected the gravity data for instrument drift, Copyright 2003 by the American Geophysical Union. tides, latitude, and lithospheric cooling. We also made free 0094-8276/03/2003GL017126$05.00 water corrections to the data. The resulting uncertainties are 29 -- 1 29 - 2 NOONER ET AL.: STRUCTURE OF OCEANIC CORE COMPLEXES Table 1. Gravity Data Uncertainties Error Source Uncertainty (mGal) Gravimeter accuracy 0.028 Tides 0.010 Latitude correction 0.071 Free water correction 0.030 Bouguer slab correction 0.004 Terrain correction 0.262 Total RMS uncertainty 0.275 unmatched depth, a Monte Carlo approach was used: nor- mally distributed zero mean random noise (with standard deviation s = 20 m) was added to the bathymetric data and the terrain correction was computed. One thousand iterations of this yielded an average standard deviation of 0.262 mGal, which was adopted as the terrain correction uncertainty. This provides a better statistical estimate of uncertainty than simply computing the gravity effect of a 20 m slab, which would give an uncertainty value of about 2.4 mGal. [8] The complete Bouguer anomaly is shown in Figure 2. The RMS uncertainty in the measurements and corrections is summarized in Table 1. The total uncertainty (0.275 mGal) is Figure 1. Bathymetry map with the location of seafloor dominated by the imperfect terrain correction (0.262 mGal), gravity sites shown as white circles. Alvin dives are demonstrating the need for more detailed bathymetry in high numbered. Notice the corrugations running from east to precision seafloor gravimetry. However, the results of this west near dive 3642. study were not strongly influenced by this issue. summarized in Table 1. Further details can be found in 3. Gravity Modeling 1 Appendix 2 of the supporting material . The accuracy in [9] Although difficult to model, small-scale features calculating the complete Bouguer anomaly from seafloor undoubtedly affect our ability to interpret the data. For gravity data is greatly dependent on how well the shape of instance, there is probably a continuum of density values as the seafloor itself is known. In a region like the MAR 30° N well as a complicated density interface boundary, such as a area, the terrain is often rugged and steep, making a precise steepening or shallowing dip angle with depth. Due to these terrain correction difficult. The regional bathymetry was factors and to the inherent non-uniqueness of gravity, we gridded with a 100 m spacing [Blackman et al., 1998], approached the problem by looking only at simple, end which effectively smoothes the terrain, causing discrepan- member model geometries that neglect these second order cies between the actual depth and that predicted by the grid. effects. Thus we tested four types of models: homogeneous To minimize this problem, depth was measured at the density, one-fault, cylindrical plug, and wedge. gravity sites with a Paroscientific 410k pressure gauge which has demonstrated a repeatability of 3 cm in relative depth [Sasagawa et al., 2003]. The free water corrections and the slab component of the terrain correction were made using these measured instrument depths. The resulting uncertainty in the slab correction is 0.004 mGal, using 2900 kg/m3 as the reference density. We estimate the uncertainty in the free-water anomaly to be 0.083 mGal, whereas previous seafloor gravity studies reported uncertainties of 0.30 mGal or greater [Luyendyk, 1984; Hilde- brand et al., 1990; Holmes and Johnson, 1993; Ballu et al., 1998; Cochran et al., 1999]. [7] The RMS difference between the measured depths and those obtained from the bathymetry grid was 37 meters (150 m maximum). Shifting the survey points 100 meters west relative to the bathymetry reduced the maximum difference to about 30 m, with an RMS difference of 20 m. To determine the error in the terrain correction due to this 1Supporting material is available via Web browser or via Anonymous Figure 2. This figure shows one model fit to the seafloor FTP from ftp://ftp.agu.org, directory ’’‘‘ apend(Username = ‘‘anonymous’’, Password = ‘‘guest’’); subdirectories in the ftp site are arranged by and sea surface data. The dip angles bounding the high- paper number.

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